Certain forms of ionization detectors measure ionization of gaseous matter within a gas chamber into which a sample gas is delivered, utilizing a radiation source producing beta emission for the ionization of the sample. Such beta emitters find use in electron capture detectors and helium ionization detectors; for example, electron capture detectors are widely used in the field of pesticide residue analysis.
These prior detector cells typically contained a radioactive source such as Ti.sup.3 H or .sup.63 Ni as the internal beta emitter. Although the .sup.63 Ni emitter has a lower emitter activity, hence a lower sensitivity and smaller linear range, and a higher cost than the well known Ti.sup.3 H source, it has a distinct advantage in use with detectors for operation at relatively high temperatures.
As explained in U.S. Pat. No. 3,601,609, issued Aug. 24, 1971 to W. L. Yauger, entitled "Ionization Detection Device Using A Nickle-63 Radioactive Source", the United States Atomic Energy Commission limits the temperature of operation of detectors using tritiated foils as a source to operating temperatures of about 220.degree.-225.degree. C since tritium emanation from such tritiated foil sources reaches unacceptable levels above this temperature. However, oftentimes it is desirable to operate the detector at a much higher temperature. For example, electron capture detectors are widely used in the field of pesticide residue analysis requiring such higher temperatures. However, the AEC has imposed temperature limit of about 220.degree. C, which causes chronic condensation contamination of the foil source during use. This condensation is very deleterious to performance since the tritiated foils are low energy beta emitters and any obstruction will seriously inhibit the radioactive ionization of the sample gases. For this reason, it is desirable to operate at temperatures above 250.degree. C, for example at 350.degree. C. This higher temperature operation is permitted by the AEC with the expensive .sup. 63 Ni sources, whereas the use of tritium (.sup.3 H) emitters has been heretofore forbidden due to their excessive tritium emanation rate at such high temperatures.